The gain was not significant—it rose from 3.9 percent to 4 percent—but it may come to represent a turning point of sorts for a company that started 2010 with a 15.7 percent share of the mobile market. Microsoft's share has plunged as it abandoned its previous platform, Windows Mobile, and Windows Phone hasn't been attractive enough to stop the rise of Apple's iOS and Google's Android platforms.

But it would be premature to celebrate a minuscule gain as indicative of a wholly positive direction for the platform. The Nokia Lumia 900, introduced in April to American consumers, is probably responsible for the bump, but sales have been lackluster overall.

Furthermore, Microsoft abandoned current Windows Phone owners by announcing that Windows Phone 7 phones, including the Lumia 900, will not be eligible for an upgrade to Windows Phone 8. Consumers have no incentive to buy a Windows Phone at the moment because they know the technology will be outdated by the end of the year.

It's not all bad for Windows Phone. The next generation of Windows Phones will be built on the same core technology as desktop Windows 8, which should help developers create mobile apps and improve the Windows Phone content ecosystem. However, with all current Windows Phone offerings on an inexorable slide to antiquity, the platform will continue to shed market share until the release of the next generation Windows Phone software.

Other developments worth nothing in ComScore's latest numbers:

Android's market share, although massive, is flattening in the U.S. for reasons we discussed in our State Of Android report.

Blackberry is dead in the water; its market share dropped two percentage points between February and May alone.

Apple has been on a tear since the release of the iPhone 4S; it picked up 1.7 percentage points over the three month period.